1. Field of the Invention
This invention relates to the solution polymerization of ethylene. More particularly, this invention relates to the recovery of ethylene for recycle purposes from such a process.
2. Description of the Prior Art
Heretofore, polyethylene has been formed by polymerizing ethylene while dissolved in a solvent such as hexane. The resulting single liquid phase solvent solution (solution or single phase solution) also contains a polymerization catalyst. The polymerization reaction is carried out in the single liquid phase containing at least ethylene and catalyst dissolved in a solvent. Optionally, one or more co-monomers can be present. For sake of clarity and brevity, this invention will be described in respect of polymerizing ethylene alone to form linear high density polyethylene (HDPE). However, one or more co-monomers can be employed in the practice of this invention.
The polymerization of the ethylene monomer is carried out using a series of stirred reactors followed by a tubular (plug flow) reactor. The plug flow reactor is employed upstream of an adsorber to accomplish product uniformity with a uniform residence time distribution for the reactants in that reactor. By “plug flow,” what is meant is substantially uniform fluid velocity distribution across a transverse cross-section of a reactor, and maintenance of that flow as that fluid passes longitudinally through the reactor from its entrance to its exit. This gives all portions of that process fluid essentially uniform residence time in the reactor.
Downstream of the last (plug flow) reactor a catalyst deactivator is injected into the solution, and the resulting mixture introduced into an adsorption pressure vessel which adsorbs various compounds and decomposition components from the single phase solution. The polymerization reaction is carried out at an elevated temperature of from about 150 to about 280 degrees Centigrade (C) at a pressure of from about 2,000 to about 4,000 psig. The adsorption step of this process is also carried out in this high pressure range.
The adsorbent material used in this pressure vessel is typically a particulate material. These particles adsorb from the single phase liquid solution catalyst, various catalyst moieties, and by-products (residue) from the decomposition of the catalyst deactivator. The adsorbent is typically activated alumina particles such as alumina spheres about 1.7 millimeters in diameter.
The HDPE process must be carried out in a single phase solution. If two phases (a polymer rich phase and a separate solution rich phase) is allowed to form in the reaction zone or in the adsorption zone, a phenomenon known in the art as “frosting” or “two-phasing” occurs wherein solid polymer forms and separates out from the single phase solution. Although two-phasing is desired downstream of the reactors and adsorbers, it is not desirable in the interior of the reactors and adsorbers because solid polymer that comes out of solution and deposits in the equipment in those zones.
Process conditions such as temperature, pressure, and mass composition of the single phase solution stream can determine whether the stream will stay in the single phase or move toward two-phasing. For example, an elevated ethane content can induce two-phasing. If two-phasing is allowed to continue unchecked, the reactors, adsorbers, and/or associated equipment in which the two-phasing occurs will eventually plug up with solid polyethylene deposits thereby requiring shutdown of the plant, and clean up of at least the affected equipment, a costly event in terms of lost production and clean-up expenses.
Downstream of the adsorption step two-phasing is deliberately induced in a separation unit. This is accomplished by way of a series of de-pressurization steps to form the desired, at this point in the process, formation of distinct polymer rich and solution rich phases. In this separation step, the polymer rich phase is physically separated from the solvent rich phase. The separated polymer rich phase is processed further to provide the desired polyethylene product of the process.
The separated solvent rich phase is processed to remove impurities and to separate solvent from un-reacted ethylene so that the recovered, separate solvent and un-reacted ethylene streams can be recycled to and reused in the afore described ethylene polymerization process.
Ethane is one of the impurities formed in the foregoing polymerization process. This impurity, in part by way of the ethylene recycle stream, tends to build up in concentration in the fluid circulating in the polymerization process, and, if allowed to build up to a substantial extent, will cause polyethylene prematurely to come out of the single phase solution and cause undesired polymer plugging problems upstream of the aforesaid separation unit.
To prevent undesired build up of ethane in the polymerization process, a purge stream has heretofore been taken from the ethylene recycle stream upstream of the last step employed to remove impurities from that recycle stream before it is returned as feed to the polymerization process. In order to keep the ethane content of the polymerization process below a level where solid polyethylene prematurely separates out from the single phase solution, a purge steam of substantial volume was removed from the process. A substantial amount of un-reacted ethylene was lost with this purge stream.
It is desirable to minimize the amount of purge stream taken from the polymerization process while still maintaining the ethane content in the overall polymerization process at a level below that at which solid polyethylene forms and comes out of the single phase solution upstream of the separation step. It is also desirable to minimize the amount of ethylene lost from the polymerization process, and to improve the purity of the un-reacted ethylene that is recovered and recycled to the polymerization process. This invention accomplishes all of these desirables at the same time.